feat(organize): separate source dir from destination dir (从源目录整理到目的地目录)

Previously organize only exposed a single 'target dir' that was actually
the destination, conflating 源目录 (where files to organize live) with
目的地目录 (where organized files go). Add an explicit source directory:

- OrganizeOptions gains SourcePath; DestPath replaces the old TargetPath
  (destination) for clarity. New organize.source_dir setting + source_path
  request override; resolveSourceRoot falls back to library path.
- OrganizeLibraryWithOptions only organizes media located under the source
  root, so operators can point at a specific download/staging folder and
  organize into a distinct destination.
- Handler accepts source_path/dest_path (target_path kept as a deprecated
  alias for the destination, backward compatible).
- Settings page splits into 整理源目录(待整理) + 整理目的地目录; Tools organize
  panel exposes 源目录 + 目的地目录 inputs with clear copy.

Defaults are unchanged (source = destination = library path) so existing
setups behave identically. Adds a regression test that organize is scoped
to the source directory.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal/handler/organizer.go

@@ -11,8 +11,13 @@ import (
 )
 
 // organizeReq carries optional per-request overrides. 留空则沿用系统设置。
+//
+// source_path = 源目录（待整理），dest_path = 目的地目录（整理输出）。
+// target_path 为 dest_path 的向后兼容别名。
 type organizeReq struct {
-	TargetPath   string `json:"target_path"`
+	SourcePath   string `json:"source_path"`
+	DestPath     string `json:"dest_path"`
+	TargetPath   string `json:"target_path"` // deprecated alias for dest_path
 	TransferMode string `json:"transfer_mode"`
 }
 
@@ -21,7 +26,14 @@ type organizeReq struct {
 func bindOrganizeOptions(c *gin.Context) service.OrganizeOptions {
 	var req organizeReq
 	_ = c.ShouldBindJSON(&req)
-	opts := service.OrganizeOptions{TargetPath: strings.TrimSpace(req.TargetPath)}
+	dest := strings.TrimSpace(req.DestPath)
+	if dest == "" {
+		dest = strings.TrimSpace(req.TargetPath)
+	}
+	opts := service.OrganizeOptions{
+		SourcePath: strings.TrimSpace(req.SourcePath),
+		DestPath:   dest,
+	}
 	if m := strings.TrimSpace(req.TransferMode); m != "" {
 		opts.TransferMode = service.TransferMode(m)
 	}